A known image forming apparatus is configured to, during continuous printing, reduce electrostatic discharge from occurring between a trailing end portion of a sheet and a photosensitive drum and reduce inconsistency in density due to a change in the surface potential of the photosensitive drum. Specifically, in the image forming apparatus, a transfer bias is turned off before the trailing end portion of the sheet passes a transfer position between the photosensitive drum and a transfer roller, thereby the electrostatic discharge is reduced from occurring between the trailing end portion of the sheet and the photosensitive drum when the trailing end portion of the sheet passes the transfer position.
When the transfer bias is turned off in such a case, there is a problem that the absolute value of a surface potential in a portion (affected by the turning off of the transfer bias) of the photosensitive drum may get higher than that in other portions of the photosensitive drum, resulting in inconsistency in density. Specifically, even when the portion whose absolute value has become great is exposed, the portion should have greater surface potential than the other portions. Developer does not remain on the portion and thus inconsistency in density occurs.
In the known image forming apparatus, when an area of the photosensitive drum corresponding to a period of time during which the transfer bias is off (or an area of the photosensitive drum passing the transfer position while the transfer bias is off) reaches the developing roller, the developing bias is made slightly greater than the normal developing bias. Through the application of the greater developing bias, toner can be supplied to the portion of the photosensitive drum where the surface potential has become great (or the exposed portion thereof). Thus, toner can remain on the portion where the surface potential has become great and inconsistency in density can be reduced.